1. Technical Field
This invention relates generally to an apparatus and method for determining the amount of solid particles of particulate matter in exhaust gas discharged from an internal combustion engine, and more particularly, to such an apparatus and method for measuring the mass, size and number of particles comprising solid and volatile particulate matter emitted from combustion sources in real-time.
2. Background Art
Particulate matter (PM) emitted from combustion sources, particularly diesel engines, is typically composed of volatile and solid fractions. The solid fraction, often referred to as xe2x80x9csootxe2x80x9d, consists mainly of carbon and a small amount of inorganic ash. The volatile fraction consists of unburned and partially burned fuel and lubricating oil, and sulfur compounds. Dry soot particles are formed in the combustion chamber of an engine while most of the volatile fraction enters the particle phase from the gas phase as the exhaust cools. The particle phase of the volatile fraction consists of precipitated liquid particles and precipitated liquid that attach to preexisting solid particles. Hence, particulate matter is a combined measure of solid and volatile fractions. It is important to be able to distinguish between the two components because the mechanism of their formation and control are different.
Several systems and methods have been proposed for the measurement of total (solid and precipitated volatile fractions) particulate mass. For example, U.S. Pat. No. 6,148,656, granted to Leo Breton on Nov. 21, 2000, for a REAL-TIME ON-ROAD VEHICLE EXHAUST GAS MODULAR FLOW METER AND EMISSIONS REPORTING SYSTEM, describes an on-road system for measuring emissions from a vehicle in real-time. The system provides a chemical analysis and measure of total particulate matter emitted, which can be expressed in grams per mile. However, this system is not capable of identifying the solid particles separately attributable to the solid and volatile fractions of the particulate material, nor in identifying the size distribution and number of solid particles.
The measurement and identity of the solid and volatile fractions of particulate matter in real-time would be of great value to engine developers and emission researchers focusing on ways to characterize and reduce soot emissions. Furthermore, if the solid particle measurement is conducted with the volatile fraction removed, in parallel with a measurement of total particulate matter (soot and volatile fractions), one would also be able to determine volatile PM emissions in real-time, by taking the difference between the solid PM and total PM. Currently, no available method is capable of facilitating the measurement of solid particle mass, number and size emissions in real-time. Most available methods rely on chemical work-up, and they are slow, notwithstanding the elaborate system proposed by Breton as discussed above, and often take several hours to several days from the time of initial measurement. Aside from particle mass, the results from available methods are not capable of establishing particle number and size.
The present invention is directed to overcoming the problem of separately identifying the source of particulate matter in emissions attributable to the solid and the volatile fractions of the particulate matter. It is desirable to have an apparatus and method that can remove the volatile fraction from the particulate matter, leaving only particles from the solid fraction for measurement of mass, size distribution and number. It is also desirable to have such an apparatus and method that can provide rapid cooling prior to the measurement of solid particles to inhibit the formation of sulfuric acid particles during cool down from a higher exhaust temperature.
In accordance with one aspect of the present invention, a method for real-time measurement of the mass, size and number of solid fraction particulate matter in exhaust gas discharged from an internal combustion engine includes discharging exhaust gas from the engine over a predefined period of time while operating the engine in a predetermined mode. A first sample of the discharged exhaust gas is continuously collected throughout the predefined period of time and passed through a catalytic stripper that removes at least about 90% of the volatile fraction from the first gas sample. The first sample is then cooled to a temperature less than about 52xc2x0 C. and the size and number of solid particles contained in the first sample of exhaust gas are measured over the predefined period of time. The measured size and number of solid particles are representative of the distribution of the solid fraction particulate matter discharged from the engine over the predefined period of time. The solid particles are collected over the predefined period of time and the mass of the collected solid particles measured. The measured mass of solid particles contained in the first sample is representative of the total mass of the solid fraction of the particulate matter discharged from the engine over the predefined period of time.
Other aspects of the method embodying the present invention include continuously collecting a second sample of the exhaust gas discharged from the internal combustion engine over a like predefined period of time, and cooled to a temperature sufficient to precipitate substantially all of the volatile fraction from the gaseous phase to a liquid phase. The solid fraction of the particulate matter and the precipitated liquid phase of the volatile fraction formed throughout the like period of time are collected and measured. The measured mass is representative of the total mass of the solid fraction and the liquid particles of the volatile fraction comprising the particulate matter discharged from the engine during the like period of time.
Another aspect of the method embodying the present invention includes calculating the difference between the mass of the first and second samples of exhaust gas, the difference being representative of the volatile fraction particulate matter contained in the exhaust gas discharged from the engine.
In accordance with another aspect of the present invention, an apparatus for real-time measurement of the mass, size and number of solid fraction and volatile fraction particulate matter in exhaust gas discharged from an internal combustion engine includes a catalytic stripper in fluid communication with an exhaust system of the engine. The catalytic stripper is adapted to remove at least about 90% of the volatile fraction particulate matter from the exhaust gas and pass at least about 95% of the solid fraction particulate matter of the exhaust gas. The apparatus further includes a micro-dilution tunnel in fluid communication with the catalytic stripper. The micro-dilution tunnel is adapted to mix air having a temperature of less than that of the exhaust gas with the exhaust gas and form a mixture of air and exhaust gas having a temperature of less than 52xc2x0 C. The apparatus further includes a particle sizer and a particle counter in fluid communication with the micro-dilution tunnel. A means for continuously collecting particulate matter contained in the mixture of air and exhaust gas over a predefined period of time is also provided, along with a means for measuring the total mass of the particulate matter collected over the predefined period of time.
Other aspects of the apparatus embodying the present invention include a dilution tunnel interposed between the exhaust system of the engine and the catalytic stripper. The dilution tunnel is adapted to mix air with the exhaust gas discharged from the engine and provide a mixture of air and exhaust gas to the catalytic stripper.